The present invention relates generally to failure analysis and qualification testing of integrated circuits (ICs) or microelectromechanical (MEM) devices, and specifically to an apparatus and method for analyzing an IC or MEM device for any open-circuit or short-circuit defects therein based on localized heating of electrical conductors or electrically-active members therein using a focused and scanned laser beam.
Open-circuit and short-circuit defects in electrical conductors or electrically-active members in ICs or MEM devices can result in major yield and reliability problems. A capability for localizing and identifying these types of defects is important for analyzing ICs or MEM devices to determine failure mechanisms therein, for qualifying the ICs or MEM devices as known-good devices, and for implementing corrective action during fabrication to minimize the occurrence of such defects. Efficient and reliable detection for open-circuit and short-circuit defects will become of increasing importance as the number of interconnection levels and the length of interconnections increase with the development of new generations of ICs and MEM devices having on-board integrated circuitry. Efficient and reliable detection of short-circuit defects is also important for MEM devices where short-circuiting of electrically-active members can occur as a result of fabrication defects or stiction (i.e. adhesion of one or more electrically-active members with an adjacent electrically-active member or with a supporting substrate whereon the MEM device is formed.) Stiction in MEM devices can arise during fabrication of the devices (e.g. during an etch release step wherein one or more layers of a sacrificial material encapsulating the MEM device are removed to free the device for operation), or during use whenever surfaces of mechanical or electrically-active members of the MEM device come into contact.
Present ICs and MEM devices having on-board circuitry generally employ multiple levels of patterned metallization that can obscure lower electrical conductor levels, thereby complicating failure analysis or qualification testing from a device side (i.e. a top side) whereon the levels of patterned metallization are formed. Additionally, flip-chip packaging of ICs or MEM devices can make device-side analysis difficult, if not impossible. As a result, there is a need for the development of analysis methods that can operate on both a device-side and a substrate-side (i.e. a bottom side) of the ICs or MEM devices to be tested, thereby facilitating the detection of any open-circuit and short-circuit defects in the ICs or MEM devices.
Unfortunately, presently available substrate-side analysis methods are not totally effective in localizing open and shorted conductors. Furthermore, the presently available methods can be time consuming; they can yield a great deal of superfluous information; and they can provide only indirect evidence of open-circuit and short-circuit defects. What is needed is a rapid, sensitive method for analyzing ICs and MEM devices, with or without on-board circuitry, for open-circuit and short-circuit defects that operates under any mounting configuration.
An advantage of the apparatus and method of the present invention is that a high sensitivity for analyzing an IC or MEM device for any short-circuit defects therein can be realized by biasing the IC or MEM device with a constant-current source and measuring a change in a variable voltage of the source in response to a change in power demand by the IC produced upon irradiating a particular defect with a focused and scanned laser beam.
Another advantage is that, in some embodiments of the present invention for analyzing MEM devices, an induced voltage can be produced directly in the MEM device so that the constant-current source can be omitted.
Yet another advantage of the apparatus and method of the present invention is that any short-circuit defects can be located within an IC or MEM device from either a device side (i.e. a top side) or a substrate side (i.e. a bottom side) thereof.
Still another advantage of the present invention is that an entire die can be examined in a single image to locate any short-circuit defects therein.
Yet another advantage of the present invention is that it is nondestructive and can be used for qualification testing of ICs and MEM devices to locate any short-circuit defects therein. In the case of MEM devices when the short-circuit defects result from stiction, post-processing can be used to remove the stiction and thereby eliminate the short-circuit defects.
These and other advantages of the method of the present invention will become evident to those skilled in the art.
The present invention relates to a thermally-induced voltage alteration (TIVA) apparatus and method for analyzing a microelectromechanical (MEM) device formed on a substrate for any short-circuit defects therein. The TIVA apparatus in one embodiment thereof comprises a laser producing a laser beam; means for focusing and scanning the laser beam to irradiate a portion of the MEM device and thereby generate an induced voltage at the location of any short-circuit defects therein, the focusing and scanning means further providing a position signal to indicate the location of the laser beam on the irradiated portion of the MEM device; and means, comprising inputs of the induced voltage and the position signal, for indicating the location of each short-circuit defect within the irradiated portion of the MEM device. Although the substrate holding the MEM device can be formed of any suitable material (e.g. a semiconductor or a glass), the present invention is particularly suitable for analyzing MEM devices comprising a plurality of stacked layers of polycrystalline silicon formed on a silicon substrate.
The means for focusing and scanning the laser beam can comprise, for example, a scanning optical microscope. The means for indicating the location of each short-circuit defect within the irradiated portion of the MEM device can comprise, for example, a display or a computer, and can further include an image processor.
The TIVA apparatus can further comprise a stage for holding the MEM device, with the stage optionally including means for heating or cooling the MEM device. Additionally, the TIVA apparatus can include a switch matrix, connected between the MEM device and the means for indicating the location of each short-circuit defect within the irradiated portion of the MEM device, for selecting among a plurality of electrical connections to the MEM device (e.g. to select among a plurality of electrically-active members in the MEM device). The switch matrix can be operated manually or by computer control. The TIVA apparatus can also comprise a voltage amplifier, connected between the MEM device and the means for indicating the location of each short-circuit defect within the irradiated portion of the MEM device, for amplifying the induced voltage. The voltage amplifier, which provides an increased sensitivity for detection of any short-circuit defects in the MEM device, can be either alternating-current (ac) coupled or direct-coupled (dc) coupled. Finally, the TIVA apparatus can comprise a photodetector (e.g. a silicon or germanium photodetector) for detecting a portion of the laser beam reflected or scattered from the MEM device. An electrical signal generated by the photodetector can be provided to the indicating means to generate a reflected-light image of the irradiated portion of the MEM device. This reflected-light image can then be superposed in the indicating means with a generated image of each short-circuit defect to provide for precise location of the short-circuit defects within the MEM device.
The laser in the TIVA apparatus can be selected to provide a wavelength for the laser beam that is in the range of 0.3 xcexcm to 2.5 xcexcm, depending upon whether the laser beam is to irradiate the MEM device from a top side of the substrate proximate to the MEM device, or whether the laser beam is to irradiate the MEM device from a bottom side of the substrate distal from the MEM device with the laser beam being transmitted through the substrate. For substrate-side irradiation when the substrate comprises, for example, silicon a neodymium:aluminum:garnet (Nd:YAG) laser operating at 1.32 xcexcm, or alternately a neodymium:yttrium-vanadium-oxide (Nd:YVO4) laser operating at 1.34 xcexcm can be used.
The TIVA apparatus in another embodiment thereof comprises a stage for holding the MEM device; a laser generating a laser beam; a scanning optical microscope (SOM) for focusing and scanning the laser beam to irradiate a portion of the MEM device and thereby generate an induced voltage at the location of any short-circuit defect therein, with the scanning optical microscope further providing a position signal to indicate the location of the laser beam on the irradiated portion of the MEM device; and a display for receiving inputs of the induced voltage and the position signal to indicate the location of each short-circuit defect within the irradiated portion of the MEM device. In this embodiment of the present invention, the stage can optionally include means for heating or cooling the MEM device; and the SOM can include a photodetector for detecting a portion of the laser beam reflected or scattered from the MEM device. The photodetector is useful for providing an input to the display to generate a reflected-light image of the irradiated portion of the MEM device to aid in locating any short-circuit defects detected by the TIVA apparatus. The TIVA apparatus can further include a voltage amplifier connected between the MEM device and the display for amplifying the induced voltage, and a switch matrix connected between the MEM device and the display for selecting among a plurality of electrical connections to the MEM device. The laser used in the TIVA apparatus generates a laser beam having a wavelength that is generally in the range of 0.3 xcexcm to2.5 xcexcm.
In yet other embodiments of the present invention, the TIVA apparatus comprises a constant-current source connected to the MEM device to supply a constant current and a variable voltage thereto, with the voltage provided by the constant-current source to the MEM device changing in response to a change in power demand by the MEM device; a laser producing a laser beam; means for focusing and scanning the laser beam to irradiate a portion of the MEM device and thereby change the power demand by the MEM device when the laser beam irradiates any short-circuit defect in the MEM device, the focusing and scanning means providing a position signal to indicate the location of the laser beam on the MEM device; and means, comprising inputs of the changing voltage and the position signal, for indicating the location of each short-circuit defect within the irradiated portion of the MEM device. The focusing and scanning means can be provided, for example, in a conventional scanning optical microscope. The TIVA apparatus can further include a stage for holding the MEM device, with the stage optionally including means for heating or cooling the MEM device. A photodetector can be provided in the TIVA apparatus for detecting a portion of the laser beam reflected or scattered from the MEM device to generate a reflected-light image of the MEM device which can be superposed with a generated image of the short-circuit defect to aid in precisely locating the defect in the MEM device. An optional voltage amplifier can be connected between the constant-current source and the means for indicating the location of each short-circuit defect within the irradiated portion of the MEM device to amplify the changing voltage to provide a more sensitive analytical capability. An image processor can also be included in the indicating means for accumulating or averaging images to increase the sensitivity of the TIVA apparatus. Finally, a switch matrix can be connected between the constant-current source and the MEM device for controlling a plurality of electrical connections to the MEM device. This embodiment of the present invention is particularly useful for locating short-circuit defects in a MEM device formed on a silicon substrate that also includes integrated circuitry (e.g. an IC) formed on the same substrate. Here, the TIVA apparatus can be used to detect short-circuit defects in the MEM device using a laser wavelength in the range of 0.3-2.5 xcexcm. The TIVA apparatus can also be used to detect open-circuit or short-circuit defects in integrated circuitry formed on the same substrate as the MEM device. In this case, the laser can be provided with a beam having a photon energy less than a bandgap energy of the silicon substrate (i.e. a wavelength in the range of 1.2-2.5 xcexcm) to prevent the generation of photocurrents due to photogenerated carriers (i.e. electrons and holes) in the integrated circuitry.
The present invention also relates to a method for analyzing a MEM device, comprising steps for irradiating a portion of the MEM device with a focused and scanned laser beam and thereby generating an induced voltage at the location of any short-circuit defect therein; and sensing the position of the focused and scanned laser beam and the induced voltage for determining the location of each short-circuit defect in the MEM device. The sensing step can further comprise forming an image showing the location of each short-circuit defect within the MEM device. The method can additionally include one or more of the following steps: a step for heating or cooling the MEM device; a step for amplifying the induced voltage prior to the sensing step, and a step for generating a reflected-light image of the irradiated portion of the MEM device by detecting a reflected or scattered portion of the irradiating laser beam. This method is particularly useful for analyzing MEM devices which do not have integrated circuitry comprising a plurality of transistors formed on the same substrate as the MEM device.
Finally, the present invention relates to a method for analyzing a MEM device that comprises steps for supplying electrical power to the MEM device from a constant-current source having a constant current and a variable voltage that changes in response to a change in a power demand by the MEM device; irradiating a portion of the MEM device with a focused and scanned laser beam and thereby changing the power demand of the MEM device; and detecting any short-circuit defects within the irradiated portion of the MEM device by sensing a position of the focused and scanned laser beam and a change in the variable voltage from the constant-current source. The step for detecting each short-circuit defect within the MEM device can further comprise forming an image showing the location of the short-circuit defect in the MEM device. The method can additionally include steps for forming a reflected-light image of the MEM device (e.g. by detecting a portion of the laser beam reflected or scattered from the MEM device using a photodetector), and superposing the reflected light-image with the image of the short-circuit defect to form a composite image for locating each short-circuit defect in the irradiated portion of the MEM device. The method can also include a step for amplifying the change in the variable voltage prior to the detecting step. This method can be used for analyzing MEM devices with or without on-board integrated circuitry (i.e. integrated circuitry formed on the same substrate as the MEM device).
Additional advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following detailed description thereof when considered in conjunction with the accompanying drawings. The advantages of the invention can be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.